This invention relates to data acquisition systems, and more particularly to an instrumentation interface circuit for such systems which automatically configures a plurality of input and output devices with a microprocessor of a data acquisition system which monitors and/or controls the input and output devices.
Environmental control systems, surveillance systems, industrial control systems and the like, employ a plurality of different types of transducers including input devices that are monitored by a microprocessor to determine control functions that must be initiated and output devices that are controlled by the microprocessor to implement the control functions that are required. These input and output devices can be classified into four general categories, namely internally sourced input devices, internally sourced output devices, externally sourced input devices and externally sourced output devices. Internally sourced input devices include RTD temperature sensors, pressure transducers, potentiometers and dry contact inputs. Internally sourced output devices include transducers requiring a 4 to 20 milliamp output or 1 to 10 volt output and include momentary relays and latching relays. Externally sourced input devices are configured to provide 4 to 20 milliamp current inputs and 1 to 10 volt inputs to the interface circuit. Externally sourced output devices are configured to modulate a power source to conduct 4 to 20 milliamps.
Because each of these input and output devices has different characteristics, an instrumentation interface must be provided between the input and output devices and the microprocessor which monitors and controls the input and output devices. Typically, such instrumentation interface includes a plurality of analog to digital converters, a plurality of digital to analog signal converter/driver circuits and a plurality of conditioning circuits, including a conditioning circuit individually associated with each input/output device and adapted to the characteristics of the associated input/output device. The input and output devices are connected to the associated conditioning circuit which is interposed between the device and an analog to digital converter when the connected device is an input/type device and which is interposed between the device and a digital to analog signal converter/driver circuit when the connected device is an output type device. The analog to digital converter with its associated conditioning circuit converts input signals, or indications such as contact closures, into a digital sense signals for use by the microprocessor. The digital to analog signal converter/driver circuit responds to digital control signals provided by the microprocessor to provide suitable drive signals via the associated conditioning circuit for the output device connected to the conditioning circuit.
Typically, the data acquisition system includes a multiplexing arrangement which permits several of the same type of input devices to use a common analog to digital converter. However, a separate conditioning circuit has to be provided for each device because the conditioning circuit must be specific to the type of input/output device that is connected to the conditioning circuit. For example, the conditioning circuits that are connected to current input devices, must include current detecting means, and the conditioning circuits that are connected to voltage input devices must include a voltage detecting means. Similarly, each output device requires a separate digital to analog/driver circuit because the digital to analog driver circuit must be matched to the device that it drives. Accordingly, because of the large number of input/output devices employed in a data acquisition system, many different types of conditioning circuits and driver circuits must be provided to properly interface the different types of input/output devices with the microprocessor. Consequently, considerable planning is required in the layout and installation of the data acquisition system because of the need to associate the large number of input and output devices with their conditioning circuits in making the connections of each input/output device to its designated conditioning circuit.